During two shallow thermal drilling programs, thermal measurements were obtained in 56 shallow (76.2 m) and one intermediate (457.3 m) depth holes located both onshore and offshore along the southern margin of the Salton Sea in the Imperial Valley, California. These data complete the surficial coverage of the thermal anomaly, revealing the shape and lateral extent of the hydrothermal system. The thermal data show the region of high thermal gradients to extend only a short distance offshore to the north of the Quaternary volcanic domes which are exposed along the southern shore of the Salton Sea. The central thermal anomaly has an arcuate shape, about 4 km wide and 12 km long. Across the center of the anomaly, the transition zone between locations exhibiting high thermal gradients and those exhibiting regional thermal gradients is quite narrow. Thermal gradients rise from near regional (0.09 ¿C/m) to extreme (0.83 ¿C/m) in only 2.4 km. The heat flow in the central part of the anomaly is greater than 600 mW/m2 and in the two local anomalies exceeds 1200 mW/m2. The shape of the thermal anomaly is asymmetric with respect to the line of volcanoes previously thought to represent the center of the field, with its center line offset south of the volcanic buttes. There is no broad thermal anomaly associated with the magnetic high that extends offshore to the northeast from the volcanic domes. The Salton Sea thermal anomaly can be described as three nested thermal features superimposed on the regional Salton Trough heat flux: a broad region of elevated conductive heat flow with constant temperature gradients of about 0.1 ¿C/m, a central anomaly of high (0.4 ¿C/m) near-surface gradients observed through a conductive thermal cap underlain by a nearly isothermal zone of hydrothermal circulation, and two local anomalies with shallow temperature gradients as high as 0.8 ¿C/m superimposed on the central geothermal system.

These observations of the thermal anomaly provide important constraints for models of the circulation of the hydrothermal system. Thermal budgets based on a simple model for this hydrothermal system indicate that the heat influx rate for local ''hot spots'' in the region may be large enough to account for the rate of heat flux from the entire Salton Trough. ¿ American Geophysical Union 1988